Clamp-on, ultrasonic, flow measuring devices are used often in process and automation technology. They permit contactless determination of volume flow rate and/or mass flow rate of a medium in a containment, especially in a pipeline. Additionally, they have the advantage that they can be placed externally on the pipeline. Clamp-on, ultrasonic, flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
The known ultrasonic measuring devices work either by the Doppler principle or the travel-time-difference principle. In the case of the travel-time-difference principle, the different travel times of the ultrasonic measuring signals in the direction of flow, and counter to the direction of flow, of the medium are exploited. To this end, the ultrasonic measuring signals are alternatingly emitted, respectively received, in the direction of flow, and counter to the direction of flow, of the medium. On the basis of the travel-time-difference of the ultrasonic measuring signals, the flow velocity can be determined, and, with that and known diameter of the pipe, the volume flow rate of the medium, or, with known density, the mass flow rate of the medium.
In the case of the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated. On the basis of a frequency shift occurring between the ultrasonic measuring signal which was coupled into the medium and the reflected ultrasonic measuring signal, likewise the flow velocity of the medium, or the volume and/or mass flow rate, can be determined. The use of flow measuring devices working according to the Doppler principle is only possible, when, present in the medium, are air bubbles or contaminants, on which the ultrasonic measuring signals are reflected. Thus, the use of ultrasonic flow measuring devices using the Doppler principle are rather limited, compared to ultrasonic flow measuring devices using the travel-time-difference principle.
In the case of clamp-on, ultrasonic, flow measuring devices working according to the travel-time-difference principle, the ultrasonic measuring signals are radiated at a predetermined angle into the containment in which the medium is located. In order that as large a part as possible of the energy radiated from an ultrasonic transducer into the containment be received in the other ultrasonic transducer, the two ultrasonic transducers must have a defined separation from one another. The particular positions of the ultrasonic transducers on the containment depends on the inner diameter of the pipeline and on the velocity of sound in the medium. As further application parameters, with which relatively large errors can be associated, the wall thickness of the pipeline and the velocity of sound in the material of the pipeline can be named.
Depending on application, still another source of error can arise in the case of clamp-on flow measuring devices, such error coming from temperature changes of the medium or the environment. An ultrasonic transducer usable for a clamp-on flow measuring device contains at least one piezoelectric element (producing the ultrasonic measuring signals) and a coupling wedge. The coupling wedge is usually made of plastic and serves, on the one hand, to radiate the ultrasonic measuring signals at a defined angle into the pipe and, on the other hand, for impedance matching. The ultrasonic measuring signals produced in a piezoelectric element are conducted via the coupling wedge, or lead-in member, and the pipe wall, into the liquid medium. Since the velocities of sound in a liquid and in plastic differ from one another, the ultrasonic waves are refracted in passing from one medium into the other. The angle of refraction is determined by Snell's Law, i.e. the angle of refraction depends on the ratio of the propagation velocities of the two media. In general, with wedges, or lead-in members, of plastic, a good impedance matching can be achieved; however, the velocity of sound in plastic shows a relatively strong temperature dependance. Typically, the velocity of sound in plastic changes from about 2500 m/s at 25□C to about 2200 m/s at 130□C. In addition to the change of the travel time of the ultrasonic measuring signals caused by the temperature in the plastic of the coupling wedge, the direction of propagation of the ultrasonic measuring signals also changes in the flowing medium. Both changes thus affect measurement accuracy unfavorably in an ultrasonic flow measuring device working according to the travel-time-difference method. In order to keep measurement accuracy at a constant, high level, corrections of the positions of the ultrasonic transducers are, therefore, required. The angular positioning of the ultrasonic transducers is fixedly predetermined in the case of the known flow measuring devices. For the purpose of initial mounting or in the case of later application changes, it is necessary, due to the above considerations, to be able to adjust the separation of the two ultrasonic transducers with respect to one another in a defined manner. For this purpose, usually one of the two ultrasonic transducers is shifted relative to the other, until that position is found, in which the intensity of the measuring signals received from the ultrasonic transducers is maximum. Once the optimum separation of the two ultrasonic transducers has been determined in this ‘trial and error’ technique, the two ultrasonic transducers are locked tight on the pipe wall in the determined positions. This method is, understandably, relatively time consuming. For simplifying the relative movement of the ultrasonic transducers, mechanical positioning aids are used, such as millimeter scales or perforated rail. A positioning aid working with a perforated rail is described, for example, in EP 0 974 815 A1.
Added to this is the fact that some of the application parameters, which, especially in the case of a clamp-on flow measuring device, are needed for exact determination of the volume flow rate, are only in the rarest cases known with sufficient accuracy—or, the determination of these parameters is rather complicated. While the determination of the outer diameter of the pipeline scarcely offers any problems, the exact determination of the wall thickness of the pipeline can be quite problematic. In many cases, moreover, neither the velocity of sound in the material of the pipeline, nor the velocity of sound in the medium, is exactly known.